Conventionally, resistor elements are important elements and are widely used in an analog integrated circuit. Amongst various resistor elements, a resistor element made of a thin metal film (hereinafter referred to as a thin-metal-film resistor element) has become a focus of attention in recent years because its temperature coefficient of resistance (TCR), that is, a temperature dependency, is relatively low. The thin-metal-film resistor element is made of any one of materials including CrSi, NiCr, TaN, CrSi2, CrSiN, and CrSiO, for example.
In a semiconductor apparatus having the thin-metal-film resistor element, a higher sheet resistance is aimed so as to meet a demand for a high-integration design. In many cases, the thin-metal-film resistor element is formed with a thickness of 1000 Å or thinner in a semiconductor apparatus.
There are several methods for making an electrical connection to the thin-metal-film resistor element. For example, a first method is to connect a metal wiring directly to the tin-metal-film resistor element. A second method is to form an interlayer insulating film after the thin-metal-film resistor element is formed, to form a connection hole in the interlayer insulating film, and to connect a metal wiring to the thin-metal-film resistor element via the connection hole. A third method is to form a barrier film on a layer of the thin-metal-film resistor element and to connect a metal wiring to the barrier film.
Referring to FIGS. 1A-1F, an exemplary process of the above-mentioned first method is explained. FIGS. 1A-1F illustrate cross-sections of a semiconductor apparatus in respective steps of production which are sequentially conducted. Before the step of FIG. 1A, a silicon substrate 1 in a wafer state is provided. On the silicon substrate 1, formations of an element-separation oxide film 3 and other elements (not shown) including a transistor element (not shown) have been completed. Then, in the step of FIG. 1A, a BPSG (borophosphosilicate glass) film is formed on the silicon substrate 1 and is subjected to a reflow process. The BPSG film is to become a first interlayer insulating film 5 between a gate electrode of the transistor and the metal wiring.
In the step of FIG. 1B, a thin metal film 73 is formed, on the silicon substrate 1, to a thickness in a range of approximately 25 Å to approximately 500 Å. The thin metal film 73 is to form the thin-metal-film resistor element.
In the step of FIG. 1C, a resist pattern 75 is formed on the thin metal film 73. The resist pattern 75 is used to determine a formation region of the thin-metal-film resistor element. Then, by using the resist pattern 75 as a mask, the thin metal film 73 is subjected to a patterning process to form a thin-metal-film resistor element 77.
In the step of FIG. 1D, the resist pattern 75 is removed and then a metal film 79, made of an AlSiCu film, for use in wiring is formed on the surfaces of the thin-metal-film resistor element 77 and the first interlayer insulating film 5. Subsequently, a resist pattern 81 is formed on the metal film 79 so as to pattern the metal film 79 such that part of the metal film 79 remains at two opposite edges of the thin-metal-film resistor element 77.
In the step of FIG. 1E, using a wet etching technique, the metal film 79 is patterned with a mask of the resist pattern 81 so as to form a metal wiring pattern 83. For the etching treatment to the metal film 79 for use in wiring, a dry etching technique is usually used in a general production process of a semiconductor apparatus; but when the thin-metal-film resistor element 77, having a very thin thickness is laid immediately under the metal film 79, the dry etching technique cannot be used because the thin-metal-film resistor element 77 is etched with an over-etching. Therefore, it is necessary to use the wet etching technique to pattern the metal film 79.
In the step of FIG. 1F, by removing the resist pattern 81, the formations of the thin-metal-film resistor element 77 and the metal wiring pattern 83 for making an electrical connection to the thin-metal-film resistor element 77 are completed.
Referring to FIGS. 2A-2F, an exemplary process of the above-mentioned second method is explained. FIG. 2A illustrates a step in which the element-separation oxide film 3, the first interlayer insulating film 5, and the thin-metal-film resistor element 77 are formed on the silicon substrate 1 in a manner similar to the above-described steps of FIGS. 1A-1C.
In the step of FIG. 2B, a CVD (chemical vapor deposition) oxide film 85 with a thickness on the order of 8500 Å for serving as an interlayer insulating film to insulate from the metal wiring is formed on the surfaces of the thin-metal-film resistor element 77 and the first interlayer insulating film 5.
In the step of FIG. 2C, a resist pattern 87 having openings in regions corresponding to two opposite edges of the thin-metal-film resistor element 77 is formed on the CVD oxide film 85 so as to form connection holes to make a connection to the metal wiring. By using the wet etching technique, the CVD oxide film 85 is selectively removed with a mask of the resist pattern 87 and, as a result, connection holes 89 are prepared. The dry etching technique is usually used to form the connection holes 89 in a general production process of a semiconductor apparatus but when the thickness of the thin-metal-film resistor element 77 is thinner than 1000 Å, the wet etching technique is needed to form the connection holes 89, because it becomes difficult to prevent the connection holes 89 from penetrating the thin-metal-film resistor element 77.
In the step of FIG. 2D, a metal film 91 made of an AlSiCu film for use in wiring is formed on the surfaces of the CVD oxide film 85 and inner walls of the connection holes 89.
In the step of FIG. 2E, a resist pattern 93 is formed on the metal film 91 so as to pattern the metal film 91 such that part of the metal film 91 remains at two opposite edges of the thin-metal-film resistor element 77.
In the step of FIG. 2F, using the dry etching technique, the metal film 91 is patterned with a mask of the resist pattern 93 so as to form a metal wiring pattern 95. At this time, because the CVD oxide film 85 is formed underneath the metal film 91, the thin-metal-film resistor element 77 will not be etched by the dry etching process.
Subsequently, the resist pattern 93 is removed so that the formations of the thin-metal-film resistor element 77 and the metal wiring pattern 95 for making an electrical connection to the thin-metal-film resistor element 77 are completed.
Referring to FIGS. 3A-3F, an exemplary process of the above-mentioned third method is explained. FIG. 3A illustrates a step in which the element-separation oxide film 3, the first interlayer insulating film 5, and the thin-metal-film resistor element 77 are formed on the silicon substrate 1 in a manners similar to the above-described steps of FIGS. 1A-1C.
In the step of FIG. 3B, a high-melting-point metal film 97 made of materials such as TiW or the like, to serve as a barrier from the metal wiring, is formed on the surfaces of the thin-metal-film resistor element 77 and the first interlayer insulating film 5. Subsequently, a metal film 99 for use in wiring is formed on the high-melting-point metal film 97. The metal film 99 is made of an AlSi film, an AlSiCu film, or the like.
In the step of FIG. 3C, a resist pattern 101 is formed on the metal film 99 so as to pattern the metal film 99 such that part of the metal film 99 remains at two opposite edges of the thin-metal-film resistor element 77.
In the step of FIG. 3D, using the dry etching technique, the metal film 99 is patterned with a mask of the resist pattern 101 so as to form a metal wiring pattern 103. At this time, since the high-melting-point metal film 97 is formed underneath the metal film 99, the thin-metal-film resistor element 77 will not be etched by the dry etching process.
In the step of FIG. 3E, the resist pattern 101 is removed and then, by using the wet etching technique, the high-melting-point metal film 97 is selectively removed with a mask of the metal wiring pattern 103 so that a high-melting-point metal film pattern 105 is prepared. Thereby, the formations of the thin-metal-film resistor element 77, the metal wiring pattern 103 for making an electrical connection to the thin-metal-film resistor element 77, and the high-melting-point metal film pattern 105 are completed. In this step, the patterning of the high-melting-point metal film 97 with the dry etching process is difficult since the high-melting-point metal film 97 is present immediately above the thin-metal-film resistor element 77.
There is known another conventional semiconductor apparatus in which a resistor element is formed on an uppermost layer wiring electrode through an insulating film and is connected to the uppermost layer wiring electrode, although the resistor element in this case is not in a shape of a thin metal film. In this semiconductor apparatus, the resistor element and the wiring electrode seem to connect to each other with their side surfaces. This side surface connection may be extremely difficult to be materialized in a general semiconductor device manufacturing process. In addition, even if such side surface connection is materialized, when the resistor element is a thin-metal-film resistor element, a contact area between the resistor element and the wiring electrode would be considerably small, which would consequently generate a great amount of contact resistance there between. As a result, this type of semiconductor apparatus may not properly function as electrical circuitry.
In the first method, the patterning of the metal film 79 for use in wiring is difficult with the dry etching technique, in the step of FIG. 1E. This causes a problem of interfering with the high degree of circuitry integration.
Also, the thin-metal-film resistor element 77 is likely to be oxidized in general. Therefore, if the metal film 79 for use in wiring is formed under the condition that the surface of the thin-metal-film resistor element 77 is oxidized, it becomes difficult to make a good electrical connection between the thin-metal-film resistor element 77 and the metal wiring pattern 83. In a general production process of a semiconductor apparatus, a preferable electrical connection is made between the silicon substrate and the metal wiring by removing the naturally-oxidized film from the surface of the silicon substrate with a hydrofluoric acid aqueous solution, for example. The thin-metal-film resistor element 77, however, is easily etched by the hydrofluoric acid aqueous solution. Accordingly, if a removal of an oxide film is performed with the hydrofluoric acid aqueous solution before the metal film 79 is formed in the step of FIG. 1D, the thin-metal-film resistor element 77 may be etched by the hydrofluoric acid aqueous solution.
As a result, it may cause a problem of changing the resistance of the thin-metal-film resistor element 77.
The second method prepares the CVD oxide film 85 on the thin-metal-film resistor element 77 so as to allow the dry etching process to be used in the step of FIG. 2F for the patterning of the metal film 91 for use in wiring. However, the connection holes 89 for making an electrical connection between the thin-metal-film resistor element 77 and the metal wiring pattern 95 are, as described above, needed to be formed with the wet etching process in the step of 2C, which interferes miniaturization of circuitry, that is, a high degree of circuitry integration.
Further, the above-mentioned wet etching process uses the hydrofluoric acid aqueous solution, which etches the thin-metal-film resistor element 77 and therefore, additional measures are required to prevent the etching of the thin-metal-film resistor element 77 with the hydrofluoric acid aqueous solution, in which a barrier film is formed and is patterned on the thin-metal-film resistor element 77. This causes a problem of increasing the steps of the production process.
The third method allows the dry etching to be performed to etch the metal film for use in wiring, as described in the step of FIG. 3D and, in addition, it eliminates the formation of the connection holes. This method, however, needs to pattern the high-melting-point metal film 97 with the wet etching, as described in the step of FIG. 3E. In this method, the high-melting-point metal film 97 is used to form the high-melting-point metal film pattern 105 for determining a substantial length of the thin-metal-film resistor element 77. Accordingly, an area of the high-melting-point metal film 97 etched with the wet etching falls wider than a desired area, resulting in a variation of the substantial length of the thin-metal-film resistor element 77. As a consequence, the resistance of the thin-metal-film resistor element 77 is changed and at the same time, the miniaturization of the circuitry becomes difficult.
Further, in the step of FIG. 3B, the surface of the thin-metal-film resistor element 77, which has been formed before the high-melting-point metal film 97, is likely oxidized. Accordingly, such oxide film formed on the thin-metal-film resistor element 77 is needed to be removed with the hydrofluoric acid aqueous solution, in order to make a good electrical connection between the thin-metal-film resistor element 77 and the high-melting-point metal film 97. However, as described above, if a removal of an oxide film is performed with the hydrofluoric acid aqueous solution before the high-melting-point metal film 97 is formed, the thin-metal-film resistor element 77 may be etched by the hydrofluoric acid aqueous solution. As a result, it may cause a problem of changing the resistance of the thin-metal-film resistor element 77.
As such, the conventional methods inevitably require the wet etching technique due to the thinness of the thin-metal-film resistor element, and therefore, cannot avoid the variations of resistance of the thin-metal-film resistor element, resulting in interfering a high degree of circuitry integration.
Further, these conventional methods require an additional step for forming a barrier film on the thin-metal-film resistor element and an extra treatment to remove the surface oxidized film with the hydrofluoric acid aqueous solution, in order to make a good electrical connection of the thin-metal-film resistor element to the metal wiring.